I used to have that happen when I had a slow and unreliable Internet connection.

My solution was to drag the image to the next location. This seems to re-launch the download process, often getting the final tile, and starting the next batch.

You could also try the Get Tiles tool where you can indicate the limits of your project and the zoom level desired. The tool will download the tiles for you while you are free to work on something else in SBuikder.